What is GraphQL introspection?

Introspection is a built-in system that lets clients query a GraphQL server about its own schema. The server exposes meta-types like __Schema and __Type, so tools such as GraphiQL, code generators and documentation explorers can discover every type, field and directive at runtime.

Why do all the names start with double underscore?

The double-underscore prefix is reserved exclusively for the introspection system. The spec forbids user-defined types or fields from starting with __, which guarantees no naming collision between your schema and the meta-schema.

What is ofType used for?

ofType unwraps a single layer of a wrapping type. LIST and NON_NULL types have a null name and an ofType pointing to what they wrap, so to fully resolve a type like [String!]! you recurse through ofType: NON_NULL → LIST → NON_NULL → SCALAR String.

What does __typename return?

__typename is a meta-field available on every object type that returns the concrete type's name as a String. It is essential for resolving unions and interfaces on the client, letting code branch on which member type was actually returned.

Is introspection always available?

Not necessarily. It is on by default in development, but many production servers disable introspection to reduce schema disclosure and limit attack surface. If a __schema query is rejected, introspection has been turned off.

GraphQL Introspection Types

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How GraphQL describes itself

Every GraphQL server ships a meta-schema that lets clients ask the API about its own shape. This introspection system powers GraphiQL, schema documentation, client code generation and validation. This reference lists the introspection meta-types — __Schema, __Type, __Field, __InputValue, __EnumValue, __Directive — their fields, the __TypeKind enum, and the three meta-fields you can call directly inside any query.

How it works

You explore a schema by querying the __schema or __type root meta-fields. Because types can be wrapped in lists and non-null markers, __Type is recursive: wrapping kinds carry a null name and an ofType pointer:

{
  __type(name: "User") {
    name
    kind
    fields {
      name
      type { kind name ofType { kind name } }
    }
  }
}

To resolve a type like [String!]! you peel ofType one layer at a time: NON_NULL → LIST → NON_NULL → SCALAR(String). Which __Type fields are populated depends on kind: fields only appears on OBJECT/INTERFACE, enumValues only on ENUM, inputFields only on INPUT_OBJECT.

Notes and gotchas

The __typename meta-field is the workhorse for client-side union and interface resolution — it returns the concrete object type. All introspection names use a reserved double-underscore prefix that your own schema may never use. Finally, remember that introspection is frequently disabled in production, so do not build runtime features that depend on a live __schema query against a hardened API.